Dear Miss Snark,
Lets say one's agent, bless her heart, has sold the manuscript.
One is so pleased.
Lets say one wants to send a thank you.
Flowers? Candy? Subscription to the New Yorker? Hand knitted scarf? Matryoska dolls? Lavender soap?
What is customary?
I have received a red feather boa, flowers, choccies, CDs, books, and a lifesize photo of Mr. Clooney. Much as I adore getting presents (and really..a red feather boa was EXACTLY what I needed!) you only need to send a nice hand written thank you.
This is our job. We're almost as glad to sell something as you are to have it sold.